Electrical measurement systems and techniques have long been used to characterize the electrical properties of bulk materials, for example, resistivity, permittivity and permeability. These techniques have been adapted to measure characteristics of surfaces and thin films and have been combined with optical techniques for measuring further properties such as semiconductor type and concentrations and chemical bonding. Attempts to apply these electrical and optical techniques to the fine surface structures developed in semiconductor integrated circuits (ICs) have been stymied by the small scale of modern IC features, typically well below 100 nm, with the result that most measurement probes and beams average over neighboring features of the IC.
Atomic force microscopy has been developed to profile the topography of a specimen with a resolution of 10 nm and less. In a usual implementation, an atomic force microscope (AFM) includes a mechanical probe with a tip positioned at the end of a flexible cantilever. The tip is tapered to have an apex having a diameter of, for example, less than 50 or 100 nm though 5 nm is currently achievable. The sharp tip may be realized either through anisotropic etching of crystalline silicon to form sharp pyramidal tips with dimensions of a few silicon crystalline spacings or by etching a metal wire to form a conical tip. Through atomic interactions between the tip and specimen sufficient to affect the cantilever flexing, the probe tip can be made to hover a small fixed distance above the specimen as the tip is scanned over the specimen. Thereby, the specimen surface can be profiled by such a mechanical AFM with vertical and horizontal resolutions on the order of nanometers.
As described by Lai et al. in U.S. Pat. No. 8,266,718, incorporated herein by reference, atomic force microscopy has been combined with microwave measurement techniques to implement microwave impedance microscopy, which incorporates a microwave probe into the AFM cantilever tip. A conventional AFM system automatically scans the microwave tip closely adjacent a sample surface so that microwave circuitry can electrically characterize small areas of the sample and thus image the electrical characteristics of the scanned surface. Li et al. describe an improved microwave probe tip in U.S. Pat. No. 8,307,461. PrimeNano, Inc. of Santa Clara, Calif. markets the ScanWave™ module for AFMs to provide high-resolution imaging of permittivity and conductivity of materials at the nanoscale. The cantilever of this microwave impedance microscope includes both the tip and a shielded microwave strip line. The shielding reduces parasitic capacitance and thus enables measurement of very small electrical signals. The probe can be manufactured by techniques similar to those used for semiconductor integrated circuits, but these techniques are complex and should be performed in an expensive clean room.
In an alternative form of atomic force microscopy, a tuning fork substitutes for the vibrating cantilever. Instead, a probe tip is positioned at the end of one of the tongs of the fork. An oscillatory signal is applied to pair of electrodes formed on the parallel tongs to cause them to vibrate or oscillate against each other, preferably at the mechanically resonant frequency of the fork. Kim et al., in “Tip-sample distance control for near-field scanning microwave microscopes,” Review of Scientific Instruments, vol. 74, p. 3675 (2003), describe a microwave microscope having an etched tungsten probe electrode tip mounted longitudinally on the tips of one fork tine to utilize shear-force displacement. The shift of the mechanical resonance of the fork is used to track the topography of the sample and control the height of the probe tip. Kim et al. make no mention of their tuning fork improving the electrical measurements characterizing their sample or measuring electrical parameters of the sample.